Crew members flying moderate to high altitude aircraft, particularly high performance military aircraft, must be provided with a breathable gas mixture containing a higher concentration of oxygen than is available from ambient air at the altitude of flight. Traditional aircraft breathing systems employ compressed gases or liquid oxygen supply carried aboard the aircraft from which the breathable gas is derived.
In recent years, sorption techniques have been employed to produce a breathable gas mixture from ambient air. Because air at the altitudes of concern contains insufficient oxygen for crew members, sorption beds that preferentially adsorb nitrogen are employed to produce the breathable mixture. Ambient air that is compressed by an aircraft engine or an auxiliary compressor is supplied to the sorption apparatus where a sorption material, typically a molecular sieve, preferentially adsorbs nitrogen while allowing oxygen and the other components of air, principally argon, to pass through. Molecular sieve sorption beds can readily produce a product gas containing more than 94% oxygen. Appropriate molecular sieve materials are commercially available, such as type 5A produced by Union Carbide Corporation and Bayer A.G. and type 13X, also available from Union Carbide Corporation. These molecular sieves are well known in pressure swing adsorption technology.
In pressure swing adsorption technology, a sorption bed is effective in adsorbing a component of a gas when a gas mixture is supplied under pressure to the bed. Eventually, during the operation of the sorption bed, the bed becomes filled with the adsorbed component and adsorption performance declines. A sorption bed is regenerated, i.e., its adsorptive efficiency is restored, by releasing the gas pressure and flushing the bed, usually in a reverse flow direction, with some of the product gas produced by another bed. The reverse flow of oxygen encourages desorption of adsorbed nitrogen. The regeneration process thus includes a depressurizing step and a purging step.
The adsorption characteristics of a sorption bed material, i.e., the effectiveness of the adsorption over time, depend upon the pressure and temperature of the gas mixture supplied. Likewise, the desorption or regeneration characteristics of a sorption bed depend upon the pressure within the bed. For a particular sorption bed material those adsorption and regeneration characteristics can be determined by laboratory measurements.
In a typical application of the apparatus providing a breathable gas mixture for the crew of an aircraft, the inlet gas mixture is air, pressurized by and bled from one of the aircraft engines. This pressurized air is further conditioned, i.e., its temperature is reduced and aerosols (small liquid particles) and particulates filtered out, for use in crew-occupied spaces on an aircraft or in the breathable gas generation system described herein.
The quantity of bleed air available for onboard oxygen generation is limited. Moreover, its pressure and temperature is subject to relatively wide swings. In conventional apparatus, sorption beds having a capacity to accommodate the highest pressure/lowest temperature inlet gas situation are employed. While these beds can handle the maximum expected inlet gas mixture flow rate, much of the time the beds are not being fully used. The excess capacity means that those beds are relatively large and heavy, both critical disadvantages in an aircraft application.
Therefore, it would be desirable to provide a sorption bed apparatus and method that respond to changes in inlet gas temperature and pressure to optimize the performance of sorption beds. By responding to inlet gas condition changes, the size and weight of the beds could be reduced, while the performance of the smaller beds in the production of product gas from a limited supply of inlet gas is optimized.